1. Field of the Invention
The present inventions relate to a mixed powder for powder metallurgy process, an iron base sintered compact made of the same, and a manufacturing method thereof.
2. Description of the Related Art
Different from conventional manufacturing methods such as rolling, forging or casting, in the powder metallurgy process, metal powder of raw material is processed by compressed powder molding, and then sintered to obtain metal products. High melting point metal material such as W and Mo, porous material such as an oil impregnated bearing and a filter, hard-metal or cermet, or other materials that are hard to obtain by conventional melting methods can be manufactured easily using this powder metallurgy process.
In addition, as the powder metallurgy process does not require cutting, the yield rate is increased, and so the dimensional precision is high. Besides, material segregation or aeolotropy, frequent in the melting process, are rare. Provided with such various advantages, several kinds of materials that have been produced by melting process are gradually manufactured by the powder metallurgy process.
Actually, many of sintered compacts manufactured by powder metallurgy process are automobile parts. Besides, iron base sintering materials are largely used for these sintered compacts. For instance, a sintered material made principally from ion powder, by blending with graphite, copper or other micro powder and sintering to improve strength, weather resistance, wear resistance or the like, is well-known. On the other hand, in order to broaden its application domain, toughness and strength are also required, and for this object, material to which Ni, Mo, or other alloy elements are added is also used.
As representative method to obtain high strength iron base sintered compact by powder metallurgy process, premix process and prealloy method are known as basic method. The premix method consists in, first, blending evenly principal component iron powder with another metal powder to prepare mixed powder, then performing compressed powder molding of the same and, thereafter heating and sintering. This method presents an advantage of relatively easy molding, but added powder in iron powder isolates or segregates in the process before the molding, or added metal powder diffuses itself hardly during the sintering. Therefore, this method brings quality problems such as scattering of strength or dimension of sintered compacts.
On the other hand, the prealloy method consists, first, in preparing alloyed steel powder (prealloy type steel powder) wherein Ni, Mo, Cr or other alloy components are solid solved (alloyed) previously in iron, performing compressed powder molding of the same and, thereafter heating and sintering. As alloyed beforehand, the aforementioned inconvenience of the premix method will not occur. Nevertheless, if the prealloy method is adopted, compression density cannot be increased sufficiently during the compressed powder molding, as alloyed steel powder extremely harder than ordinary iron powder, making difficult to obtain high-density sintered compact. Consequently, it is impossible to make the best use of the properties of the concerned alloyed steel.